Optical recording medium

ABSTRACT

An increase in cost is minimized while realizing larger capacity.  
     An optical disc  102  includes, from the bottom of the drawing: a substrate  104 ; a first information recording layer  108  formed on the substrate  104 ; and a first light transmission layer  110  having a thickness of about 20 μm formed on the first information recording layer  108 . A second information recording layer  114  is formed on the first light transmission layer  110 , and a second light transmission layer  116  having a thickness of about 90 μm is formed on the second information recording layer  114 . A laser beam La is irradiated from the side of the second light transmission layer  116  to allow recording and/or reading of an information signal on/from the first and second information recording layers  108  and  114.

TECHNICAL FIELD

[0001] The present invention relates to an optical recording medium, inparticular, an optical recording medium capable of realizing theincrease in recording capacity at low cost.

BACKGROUND ART

[0002] As a form of optical recording media for audio and video andother optical recording media for recording various information, forexample, an optical disc, on which information signals are written byembossed pits, is widely used. The most basic form of optical disc isshown in FIG. 12. This optical disc 2 comprises an information recordinglayer 8 having a reflective film 6 on a light transmission moldedsubstrate (supporting plate) 4. A protective layer 12 made of an UVcurable resin or the like is formed thereon. For this optical disc 2, alaser beam La is irradiated on the side of the molded substrate 4 actingas a light transmission layer so as to read an information signal fromthe information recording layer 8.

[0003] A recording density of the optical disc 2 can be increased as aspot diameter of the used laser beam La is reduced. The spot diameter isproportional to: λ/NA (λ: a wavelength of a laser beam, NA: a numericalaperture of an objective lens). Therefore, in order to reduce the spotdiameter, it is necessary either to reduce the wavelength λ of the laserbeam La or to increase the numerical aperture NA of the objective lens.

[0004] As the wavelength λ of the laser beam La, a wavelength of 780 nmis used for CDs, whereas a wavelength of 650 nm is used for DVDs. Forthe future, a wavelength in the vicinity of 400 nm corresponding to ablue laser is expected for use. The spot diameter is reduced by theamount of a decrease in wavelength, so that a recording density isincreased.

[0005] On the other hand, coma aberration is increased as the numericalaperture NA is increased. In order to keep coma aberration small evenwith the increased numerical aperture NA, it is necessary to reduce athickness of the light transmission layer (the molded substrate 4)through which the laser beam La passes.

[0006] Conventionally, as the light transmission layer (the moldedsubstrate 4) of the optical disc 2, a molded product made of a resinformed by injection molding or the like is frequently used. By thisfabrication method, however, it is difficult to fabricate the moldedsubstrate 4 to have a smaller thickness than that of the currently usedDVDs with good accuracy using the current technique. More specifically,in the optical disc 2 having a structure using the molded substrate 4 asa light transmission layer as shown in FIG. 12, a technique ofincreasing a recording density with a higher NA of the laser beam La toincrease the capacity reaches the limit in the present state in view ofthe fabrication technique.

[0007] To cope with this, two approaches of improvement techniques havebeen proposed.

[0008] The first approach of improvement consists in increase incapacity by multiplication of the number of information recordinglayers.

[0009] For example, a DVD-9 has a double-layered information recordinglayer in a structure as shown in FIG. 13. More specifically, a firstinformation recording layer 32 is formed on an upper surface of a firstmolded substrate (light transmission layer) 30 and then is covered witha translucent film (translucent reflective layer) 34. In the completelysame manner, a second information recording layer 38 is also formed onan upper surface of a second molded substrate 36 (in the drawing, beingoriented downward) and is then covered with a reflective film(reflective layer) 40. The first substrate 30 and the second substrate36 are bonded with each other through transparent bonding means (lighttransmission layer) 42. By irradiation of the laser beam La, aninformation signal is read from the first and second informationrecording layers 32 and 38 through the first molded substrate 30. As aresult, the capacity can be increased by increasing the number ofinformation recording layers (doubling a substantial area of theinformation recording layers).

[0010] On the other hand, the second approach of improvement mainlyconsists in further increasing the NA of the laser beam to increase arecording density of information signals in the information recordinglayer, thereby increasing the capacity. For example, the related art isdisclosed in Japanese Patent Laid-Open Publications Nos. 1997-235638,2000-203724, and the like. Specifically, as shown in FIG. 14, a moldedsubstrate 50, which does not required to transmit any light, is formedby injection molding to have a large thickness as a base of a disc body.Next, an information recording layer 54 having a reflective layer 52 isformed on the molded substrate 50. An extremely thin (at the maximum,about 300 μm) light transmission layer 56 is deposited thereon by meanssuch as attachment of a light transmission sheet, resin coating throughspin coating or the like.

[0011] The laser beam La is irradiated on the side of the thin lighttransmission layer 56 to record and/or read information on and/or fromthe information recording layer 54. In this manner, it is possible toform the thinner light transmission layer 56 while ensuring the entirestrength on the side of the molded substrate 50. Therefore, a higherdensity (larger capacity) can be achieved by the approach of furtherincreasing the NA of the laser beam.

[0012] Under the actual conditions, however, each of the above-describedfirst and second approaches of improvement techniques still has variousproblems.

[0013] For example, in “multiplication of the number of informationrecording layers” corresponding to the above-described first approach ofimprovement, it is certain that substantial enlargement of the area ofthe information recording layers is made possible owing tomultiplication of the layers. However, access to each of the informationrecording layers is achieved through the thick molded substrate 36 as inthe conventional case. Therefore, the problem that it is hard to achievea higher density for each layer still subsists. More specifically,capacity is expected to be increased only by the amount of increase inarea achieved by multiplication of the layers.

[0014] On the other hand, for the approach of improvement that “density(capacity) for each layer is increased by increasing NA through a thinlight transmission layer” corresponding to the second approach ofimprovement, various problems still remain in a concrete way to formthin light transmission layer.

[0015] Generally, as a method for forming the thin light transmissionlayer, a method of bonding a transparent sheet onto a molded substrate(or a supporting plate), a method of applying (coating) a resin by spincoating and the like have been proposed.

[0016] However, the method of bonding a transparent sheet is likely tohave elevated fabrication cost because strict optical characteristics ofthe sheet itself and thickness accuracy are required. Moreover, even ifthe thickness accuracy of the transparent sheet is good, an uneventhickness or uneven application of an adhesive changes the opticalcharacteristics to degrade the disc characteristics in some cases. Thus,high accuracy is also required for the application of an adhesive.Accordingly, under the actual conditions, many problems remain in thismethod.

[0017] The method of forming a thin light transmission layer byapplication of a resin through spin coating specifically consists inmaking a liquid UV curable resin (liquid to be applied) flow on asurface of a molded substrate (supporting plate) while rotating themolded substrate with a spindle so that the resin is uniformly extendedin accordance with the rotation of the molded substrate to coat theresin layer. Thereafter, the coated resin is irradiated with ultravioletrays so as to be cured. According to this method, the conditions such asa thickness of the coating can be adjusted by controlling a rotationspeed of the spindle, application time and a viscosity of theapplication liquid.

[0018] However, this method has a problem in that the uniformity of thesurface of the coating film is difficult to keep in such a way that aconcentration (raise) of the application liquid is likely to occur, inparticular, in the vicinity of the outer periphery. The raise does notbecome a serious problem in the case where there is a relatively largedistance between a laser optical system and a disc such as aconventional CD or DVD. However, under the conditions where a gapbetween a laser optical system and a disc is reduced by a higher NA, itbecomes an unignorable obstacle.

[0019] In the case where a relatively thick (for example, 100 μm)coating layer is formed, in particular, a thickness or a radial width ofthe raise becomes remarkably large to penetrate into an informationrecording area in some cases. Therefore, there is a possibility that arecordable area may be decreased to impair the recording/reading ofinformation signals if no countermeasure is taken. Moreover, it alsocauses the collision of a laser optical system against a disc in somecases.

[0020] Regarding a treatment of the raise or the maintenance ofsmoothness of the coating surface, various proposals for ways of copingwith them have been made in many documents, for example, Japanese PatentLaid-Open Publications Nos. 1999-203724, 1999-86355, 1999-86356, and thelike. However, this signifies, in other words, “some measure” isnecessarily needed irrespective of a concrete method thereof.Correspondingly, a fabrication process is complicated while cost isincreased.

DISCLOSURE OF THE INVENTION

[0021] The present invention is to solve such conventional problems at atime by “a leap of idea” and has an object of providing an opticalrecording medium capable of minimizing the increase in cost whileobtaining remarkable effects in “increase in capacity.”

[0022] The inventors of the present invention have solved the abovedescribed various problems at the same time by proposing an opticalrecording medium having a structure according to the following (1).

[0023] (1) An optical recording medium comprising: a substrate; a firstinformation recording layer formed on the substrate; a first lighttransmission layer formed of a resin on the first information recordinglayer to have a thickness of 40 μm or less; a second informationrecording layer formed on the first light transmission layer; and asecond light transmission layer formed of a resin on the secondinformation recording layer to have a thickness of 170 μm· or less,wherein a laser beam is irradiated on a side of the second lighttransmission layer so as to allow recording and/or reading of aninformation signal on/from the first and second information recordinglayers.

[0024] As described above, a method of coating the information recordinglayer having a reflective layer with a resin at a small thickness by,for example, spin coating and irradiating a laser beam of a high NA onthe (thin) resin-coating layer as a light transmission layer isextremely favorable method in view of its capability of remarkablyimproving a recording density for each layer.

[0025] In the case where the thin light transmission layer “alone” is tobe formed, each of a method of bonding a transparent sheet and a resincoating method still has many problems in terms of technique and cost asdescribed above.

[0026] In the present invention, this problem is solved by a reversaland progressive idea of additionally forming the second informationrecording layer on the upper surface of the light transmission layer.

[0027] More specifically, in the case where the second informationrecording layer is further formed (transcribed) on the upper surface ofthe light transmission layer, a step of pressing the upper surface ofthe light transmission layer is involved in the process of transcribingthe second information recording layer. In the present invention, thisstep also serves as “means of ensuring the flatness of the upper surfaceof the first light transmission layer.”

[0028] In the case where a resin for forming the first lighttransmission layer is enclosed, for example, in a space interposedbetween the molded substrate (or the supporting plate) and a stamper, itis not necessary to use a transparent sheet. Moreover, poor smoothnessof the upper surface of the coating layer or the generation of a raise,which was conventionally inevitable, never occurs. More specifically,owing to the function of the stamper for transcribing the secondinformation recording layer (in other words, in the course ofrealization of further increase in capacity), various inconveniences,which are conventionally present in an inevitable manner in formation ofthe first light transmission layer, can all be eliminated.

[0029] Furthermore, in the present invention, the second informationrecording layer is also irradiated with the laser beam through thesecond light transmission layer of 170 μm or less. As a result, both thefirst and second information recording layers are capable of recordingat a high density based on a higher NA of a laser beam realized by thethin light transmission layer.

[0030] Obviously, the second light transmission layer must be formed onthe second information recording layer in the present invention (morespecifically, the invention according to the above-described (1), inparticular). Thus, in formation of the second light transmission layer,inconveniences similar to those in the conventional case (basically)remain.

[0031] Nonetheless, the present invention has satisfactory advantages.The reason for this is as follows.

[0032] Specifically, in the optical recording medium according to thestructure described in the above (1), both the first and secondinformation recording layers have such a structure that the laser beamis irradiated on the side of the thin light transmission layers.Therefore, a recording density for each layer can be increased. As aresult, the increase in capacity as a whole can be realized, as comparedwith each of the improved optical recording media which haveconventionally been proposed as shown in FIG. 13 or FIG. 14. Thus, itsadded value is extremely elevated.

[0033] Nonetheless, as described above, it is possible to easily formthe first light transmission layer on the first information recordinglayer with few technical obstacles. Therefore, the cost is substantiallyhardly increased thereby. In other words, the optical recording mediumrealizes large capacity, which has not ever been achieved, at almost thesame cost as conventional one.

[0034] Specifically, even if a similar burden in view of the step andthe cost as that on the conventional light transmission layer (the lighttransmission layer 56 in FIG. 14) is present in formation of the secondlight transmission layer, the burden is “relatively” reduced in asignificant manner as compared with a conventional product having thesame burden in fabrication of a single-layered disc. The inventionaccording to (1) by itself has a remarkable effect.

[0035] Moreover, the inventors of the present invention have developedthe following structures as described in (2) to (6) as further improvedstructures. As described below in detail, by using the structures in (2)to (6), an optical recording medium, in which any inconvenience scarcelyoccur in formation of the second light transmission layer, can beobtained.

[0036] (2) In the optical recording medium in the above-described basicinvention (1), the optical recording medium is characterized in that aportion of the upper surface of the first light transmission layer,corresponding to an outer radial portion of the information recordingarea of the second information recording layer, is biased toward thesubstrate from a surface corresponding to the information recordingarea.

[0037] (3) In the above-described (2), the optical recording medium ischaracterized in that the bias of the upper surface of the first lighttransmission layer is inclined toward the substrate in such a mannerthat a thickness of the first light transmission layer is graduallyreduced in an outer radial direction.

[0038] (4) In the above-described (2) or (3), the optical recordingmedium is characterized in that a portion of the upper surface of thesubstrate, corresponding to an outer radial portion of the informationrecording area of the first information recording layer, is biasedtoward inside of the substrate from the surface corresponding to theinformation recording area.

[0039] (5) In the above-described (4), the optical recording medium ischaracterized in that the bias of the upper surface of the substrate isinclined toward the inside of the substrate in such a manner that athickness of the substrate is gradually reduced in an outer radialdirection.

[0040] (6) In any one of the above described (1) to (5), the opticalrecording medium is characterized in that each of the first and secondlight transmission layers is an UV curable resin.

[0041] Furthermore, the present invention takes advantage of theflexibility of the structure to form the structure in (7) to (9) so asto easily obtain an optical recording medium having three or morelayers.

[0042] (7) An optical recording medium comprising: a substrate; a firstinformation recording layer formed on the substrate; a first lighttransmission layer formed of a resin on the first information recordinglayer to have a thickness of 40 μm or less; a second informationrecording layer formed on the first light transmission layer; a secondlight transmission layer formed of a resin on the second informationrecording layer to have a thickness of 40 μm or less; . . . ; an n-thinformation recording layer formed on an (n−1)-th light transmissionlayer; and an n-th light transmission layer formed of a resin on then-th information recording layer to have a thickness of 170 μm or less,wherein a laser beam is irradiated on a side of the n-th lighttransmission layer so as to allow recording and/or reproduction of aninformation signal on/from the first, second . . . and n-th informationrecording layers, and wherein n is an integer of 3 or larger.

[0043] (8) In the above-described (7), the optical recording medium ischaracterized in that at least a portion of the upper surface of the(n−1)-th light transmission layer, corresponding to an outer radialportion of the information recording area of the (n−1)-th informationrecording layer, is biased toward the substrate from a surfacecorresponding to the information recording area.

[0044] (9) An optical recording medium characterized in that two opticalrecording media according to any one of the above-described (1) to (8)are prepared and bonded to each other so that their substrates faceinside, whereby the laser beam is irradiated on both sides of the discto allow recording and/or reading of an information signal on/from therespective sides.

[0045] Since any of the multilayered optical recording media obtained bythe structures in (7) to (9) has access from the side of the lighttransmission layer which is formed extremely thin (in contrast with aconventional recording medium in which laser is irradiated through athick molded substrate), a recording density for each layer is highowing to a higher NA. Therefore, a remarkable increase in capacity ascompared with a conventional case can be realized.

[0046] Moreover, the above-described set value for a thickness of eachof the light transmission layers in the present invention, inparticular, a set value of a thickness of each layer where a thicknessof the uppermost light transmission layer (the closest to the beamincident side) is larger than those of the other light transmissionlayers is obtained in view of design of an optical system in recordingand/or reading of the optical disc and the like.

[0047] In the present invention, a concrete way of forming the first andsecond light transmission layers, the first and second informationrecording layers and the like is not particularly limited. As describedbelow, various methods are conceivable for formation of these layers. Byeach of the methods, a merit (peculiar to the present invention) can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048]FIG. 1 is an enlarged cross-sectional view of a substantial part,schematically showing a structure in the vicinity of an outer peripheralportion of an optical disc to which the present invention is applied;

[0049]FIG. 2 is a process chart showing the procedure of fabricating asubstrate;

[0050]FIG. 3 is a process chart showing the procedure of producing aresin stamper;

[0051]FIG. 4 is a cross-sectional view showing the schematic of a moldassembly for fabricating the substrate;

[0052]FIG. 5 is a cross-sectional view showing the schematic of a moldassembly for producing the resin stamper;

[0053]FIG. 6 is a longitudinal cross-sectional view schematicallyshowing various methods for coating a resin constituting a first lighttransmission layer;

[0054]FIG. 7 is a process chart showing the fabrication of a secondinformation recording layer on an upper surface of the first lighttransmission layer by the resin stamper and the shaping of the uppersurface so as to further form a second light transmission layer;

[0055]FIG. 8 is a cross-sectional view showing another exemplary methodfor ensuring the smoothness of the second light transmission layer;

[0056]FIG. 9 is a cross-sectional view showing an example where concaveportions are formed only on the upper face of the first lighttransmission layer;

[0057]FIG. 10 is a process chart showing the procedure for fabricating amultilayered optical disc having three layers;

[0058]FIG. 11 is a partially enlarged cross-sectional view showing thestructure of a double-faced multilayered optical disc to which thepresent invention is applied;

[0059]FIG. 12 is a partially enlarged cross-sectional view showing thestructure of a conventional single-layered type optical disc;

[0060]FIG. 13 is an enlarged cross-sectional view of a substantial part,showing a double-faced type optical disc obtained by bonding thestructures shown in FIG. 12 with each other; and

[0061]FIG. 14 is a partially enlarged cross-sectional view showing aconventional example of type in which a laser beam is irradiated on theside of a thin light transmission layer.

BEST MODE FOR CARRYING OUT THE INVENTION

[0062] Hereinafter, exemplary embodiments of the present invention willbe described in detail with reference to the drawings.

[0063]FIG. 1 schematically shows a cross section of an optical disc(optical recording medium) according to the first embodiment of thepresent invention. This optical disc 102 comprises, from the bottom inthe drawing: a substrate (molded substrate: supporting plate) 104; afirst information recording layer 108 formed on the substrate 104; and afirst light transmission layer 110 formed on the first informationrecording layer 108. A thickness D1 of the first light transmissionlayer 110 is about 20 μm. A second information recording layer 114 isfurther formed on the first light transmission layer 110. A second lighttransmission layer 116 is formed on the second information recordinglayer 114. A thickness D2 of the second light transmission layer 116 isabout 90 μm. The laser beam La is irradiated on the side of the secondlight transmission layer 116 so as to record and/or read an informationsignal on/from the first and second information recording layers 108 and114. In an outer peripheral portion A1 of each layer in the optical disc102, a predetermined curved pattern is formed.

[0064] Hereinafter, each part will be described in detail.

[0065] The structure of the substrate 104 and the first informationrecording layer 108 will be first described.

[0066] This structure is basically similar to that of the moldedsubstrate 50, the reflective layer 52 and the information recordinglayer 54 in a conventional example that has already been described byusing FIG. 14. As a concrete fabrication method, some methods have beenproposed at present. Herein, this is formed by the procedure as shown inFIG. 2.

[0067]FIG. 2 shows an example where recording is performed only ingrooves. The grooves correspond to concavity when a minuteconcavo-convex surface is viewed from the side close to a pickup. Thisgroove extremely slightly meanders (wobbles) and is used as an addressin recording (positional information on a blank disc). In FIG. 2, thereference numeral 120 in a heavy line denotes a portion where a wobblesignal is formed.

[0068] For convenience, in this specification, a pattern of the firstand second information recording layers 108 and 114 formed when viewedfrom the pickup in the ultimately obtained optical disc 102 is denotedas a positive pattern and a reversed pattern is denoted as a negativepattern. More specifically, the wobble signal 120 is present in a grooveportion in the positive pattern while being present in a land portion inthe negative pattern.

[0069] In FIG. 2, a photoresist layer (photosensitive material) 124 isfirst provided on a glass original disc 122 so as to be cut with alaser, thereby forming grooves (the positive pattern) with wobbles bydevelopment or the like (FIG. 2(A)). Next, an electrically conductivefilm is provided so as to form a nickel-plating layer 126 by so-calledelectrocasting (FIG. 2(B)).

[0070] Thereafter, the nickel-plating layer 126 is removed from theglass original disc 122 to perform a post-treatment, so that a so-calledmaster stamper 126M having a negative pattern is completed (FIG. 2(C)).

[0071] Normally, in the case of the type of optical disc (conventionalCDs or DVDs) in which access is made from the side of the substrate, themaster stamper 126M is used or a mother disc (positive pattern) isproduced from the master stamper 126M to produce a child stamper(negative pattern) therefrom so as to be used for production of asubstrate having the positive pattern.

[0072] In this embodiment, however, the laser beam is irradiated not onthe side of the substrate 104 but on the side of the second lighttransmission layer 116. Therefore, a mother stamper 128 having apositive pattern is produced from the master stamper 126M. This motherstamper 128 is completed by performing a surface treatment on the masterstamper 126M, performing electroplating by electrocasting, and thenremoving the electroplating (FIGS. 2(D) and (E)). The outer peripheralportion A2 of the mother stamper 128 (in a practical sense, a copythereof) is bent toward the substrate 104 so as to serve as a motherstamper for substrate 128K, which is fixed to a mold 160 as shown inFIG. 4. In FIG. 4, the reference numeral 162 denotes a stationary mold;164 for a movable mold; 166 for a spring; 168 for an outer ring; 170 fora concave portion; and 172 for a ring-shaped convex portion. A stepformed by the concave portion 170 and the ring-shaped convex portion 172maintains the bend of the outer peripheral portion A2 of the motherstamper for substrate 128K. A resin constituting the substrate 104 isinjected and molded in a space interposed between the movable mold 164and the mother stamper for substrate 128K.

[0073] Returning to FIG. 2, when the resin constituting the substrate104 is made to fill the mold 160 and is then removed while maintainingthe bend of the mother stamper for substrate 128K in this manner, thesubstrate 104 having minute concave and convex portions (informationrecording area) 132 corresponding to the negative pattern of the firstinformation recording layer 108 is completed (FIGS. 2(F) to (H)). In anouter peripheral portion A3 of the minute concave and convex portions132 on the substrate 104, an inclined plane 135 corresponding to theabove-described bend is formed. A thickness of the outer peripheralportion A3 is gradually reduced in an outer radial direction, ascompared with the part of the information recording area 132.

[0074] Herein, as shown in FIG. 3, a light transmission resin stamper140 for forming the second information recording layer 114 is separatelyproduced on the basis of the master stamper (or the child stamper) 126M.Specifically, an outer peripheral portion A4 of the master stamper (orthe copy thereof) 126M is bent to the side opposite to the side wherethe minute concave and convex portions 132 are present so as to producea master stamper for resin stamper (or a copy thereof) 126G. Thisstamper 126G is fixed to the side of the stationary mold 182 of the mold180 as shown in FIG. 5. In this state, a resin for constituting theresin stamper 140 is injected and molded between the movable mold 184 ofthe mold 180 and the master stamper for resin stamper 126G, and thenremoved. As a result, the resin stamper 140 having minute concave andconvex portions 142 corresponding to the positive pattern of the secondinformation recording layer 114 is completed (FIGS. 3(B) and (C)).

[0075] Although the resin stamper 140 is a stamper, it is greatlycharacterized in having the positive pattern (that is, the wobble signal120 is present in the groove) and a convex portion 140A projectingtoward a surface for transcription (toward the first light transmissionlayer 110) in an outer peripheral portion A5 (FIG. 3(D)).

[0076] The shape itself of the minute concave and convex portions 142formed on the resin stamper 140, for transcribing the second informationrecording layer 114, is similar to that of the minute concave and convexportions 132 formed on the mother stamper for substrate 128K fortranscribing the first information recording layer 108.

[0077] In the case where the optical disc 102 according to thisembodiment is constituted as a read-only optical disc, a reflective filmis formed as the first information recording layer 108.

[0078] Next, the first light transmission layer 110 having a thicknessof about 20 μm formed on the first information recording layer 108 willbe described.

[0079] As a way of forming the first light transmission layer 110,several methods are conceivable. Three representative methods are shownin FIG. 6.

[0080] A method shown in FIG. 6(A) belongs to a spin coating method. Aresin P for the first light transmission layer is dropped along theinclined plane 152 of a cap 150 so as to be spread over the entiresurface of the first information recording layer 108 while rotating thesubstrate 104. The separately produced resin stamper 140 described aboveis placed thereon.

[0081] In a method shown in FIG. 6(B), in the approximately center in aradial direction on the first information recording layer 108, apredetermined amount of the resin P for the first light transmissionlayer is dropped in a ring shape. The above-described stamper 140 ispressed thereon. The dropped resin P for the first light transmissionlayer is spread over the entire surface of the first informationrecording layer 108 by the pressing force.

[0082] A method shown in FIG. 6(C) belongs to an injection method. Theresin stamper 140 is placed in advance so as to be opposed to the firstinformation recording layer 108. The resin P for the first lighttransmission layer is injected into a space between them 140 and 108.

[0083] In the case of any of the methods, the function of the resinstamper 140 can simultaneously realize the “shaping” of the uppersurface of the first light transmission layer 110 and the“transcription” of the minute convex and concave portions 142 of thesecond information recording layer 114.

[0084] An UV curable type resin is used as the resin P to be sealed, andthe resin stamper 140 is made of an light transmission material. As aresult, ultraviolet rays irradiated on the side of the resin stamper 140can fix the first light transmission layer.

[0085] Namely, in a transcription method which is generally calledphoto-polymer method, ultraviolet rays are usually irradiated on theside of a resin to be cured, that is, on the side of the substrate 104in this embodiment.

[0086] In this embodiment, however, the reflective film which does notallow any beam to pass has already been formed on the first lighttransmission layer 110 on the side of the substrate 104 in the casewhere it is constituted as a read-only optical disc. In the case whereit is constituted as a recordable optical disc, the first informationrecording layer 108 such as a reflective film or an optical recordingfilm, which does not allow any beam to pass, has already been formed.Therefore, it is extremely inefficient to irradiate ultraviolet ray onthe side of the substrate 104 (although it is not impossible).Accordingly, in this embodiment, a light transmission resin is used as amaterial of the resin stamper 140. Ultraviolet rays B are irradiated onthe side of the resin stamper 140 so as to cure the first lighttransmission layer 110. Therefore, there is no waste in irradiationenergy. Moreover, it is not necessary to particularly increase the sizeof an ultraviolet ray irradiation system. More specifically, the effectcan be sufficiently obtained if a light transmission resin having atransmittance of 50% or more at the wavelength of 300 nm to 400 nm isused.

[0087] The present invention does not limit any specific fabricationmethods. To cope with a similar problem, a fabrication method, in which,for example, the substrate 104 serving as a member on whichtranscription is performed can be heated by some sort of method and thefirst light transmission layer 110 is a thermoset resin, is effective inthat the ultraviolet ray irradiation on the member on whichtranscription is performed is not needed.

[0088] In the industry (the field of optical recording media), however,equipment or know-how for curing an UV curable type resin withultraviolet ray irradiation is well provided. In consideration of thepossibility of low-cost fabrication and the reliability or the degree offreedom in formation of the minute concavity and convexity on thesurface for transcription of the stamper and in formation of a biasedshape on the outer peripheral side (in particular, for thermaldeformation), it is preferred a fabrication method wherein a stamper ofa “light transmission resin” which is excellent in formability isformed, and the first light transmission layer made of the UV curabletype resin is cured by ultraviolet ray irradiation from the side of thestamper.

[0089] As shown in FIGS. 7(A) and (B), when ultraviolet rays areirradiated on the side of the resin stamper 140 to fix the first lighttransmission layer 110 in the above-described manner, minute concave andconvex portions 142 corresponding to the negative pattern of the secondinformation recording layer 114 are simultaneously formed on its uppersurface. Therefore, when the resin stamper 140 is removed in this state,the minute concave and convex portions 142 corresponding to the negativepattern of the second information recording layer 114 remains on theuppermost part.

[0090] On the upper surface of (the negative pattern of) the secondinformation recording layer 114, a translucent reflective film is formedin the case where it is constituted as a read-only optical disc. In thecase where it is constituted as a recordable optical disc, a recordingfilm or a dielectric film is formed in a translucent state so as toserve as a second information recording layer which does not allow beamto completely pass. They both have minute concavity and convexity as thesecond information recording layer 114 having the positive pattern whenviewed from the side of the pickup.

[0091] On its upper surface, the second light transmission layer 116 ofabout 90 μm is formed. Several methods are conceivable as a way offorming the second light transmission layer 116. Herein, as shown inFIGS. 7(C) and (D), the fabrication by spin coating is employed.

[0092] Now, the shape of the outer peripheral portion of each layer willbe described in detail.

[0093] As already described in the drawings so far, in this embodiment,the portion A3 on the upper surface of the substrate 104 correspondingto the outer radial portion of the information recording area S1 (theminute concave and convex portions 132) of the first informationrecording layer 108 is biased toward the inside of the substrate 104 sothat a thickness of the substrate 104 in the portion A3 is smaller thanthat in the information recording area S1.

[0094] More specifically, the bias of the substrate 104 is formed by thebending shape of the outer peripheral portion A2 of the mother stamperfor substrate 128K. The inclined plane 135 is formed to incline towardthe inside of the substrate 104 so that the thickness of the substrate104 is gradually decreased in the outer radial direction.

[0095] On the other hand, a portion A6 corresponding to an outer radialportion of the information recording area S2 of the second informationrecording layer 114 on the upper surface of the first light transmissionlayer 110 is also biased so that the first light transmission layer 110is gradually biased toward the substrate 104 in an outer radialdirection from the portion of the information recording area S2. Morespecifically, as shown in FIGS. 3 and 7, the convex portion 140A isformed in a projecting shape in the outer peripheral portion A5 of theinformation recording area S2 in the second information recording layer114 of the resin stamper 140 so as to form the basis. The first lighttransmission layer 110 has an inclined plane 137 gradually biasingtoward the substrate 104 in an outer radial direction.

[0096] Therefore, if the second light transmission layer 116 is formedby spin coating on the first light transmission layer 110 on which thesecond information recording layer 114 is transcribed, the raise 160,which is conventionally inevitably present when the resin coating isperformed by spin coating, is entirely absorbed by the inclined plane137 in the outer peripheral portion of the first light transmissionlayer 110. Therefore, even if the raise 160 itself is present, it doesnot project beyond the information recording area surface of the secondlight transmission layer 116 (see FIGS. 7(D) and 1).

[0097] Accordingly, even without any post-treatments, the optical disc102 without any particular inconvenience can be fabricated.

EXAMPLE 1

[0098] Herein, an experimental example for prevention of generation of araise on the upper surface of the second light transmission layer 116due to a bending shape of the outer peripheral portion (outer portion)of each layer will be described.

[0099] In this experiment, (as described above), the substrate 104 andthe resin stamper 140 were fabricated by injection molding. The motherstamper for substrate 128K and the master or child stamper for resinstamper 126G, which were attached to the molds for molding 160 and 180,had a track pitch of 0.3 μm for groove recording.

[0100] As the substrate 104, using a polycarbonate resin (H4000-N282fabricated by Mitsubishi Engineering-Plastics Corporation), thesubstrate 104 having an outer diameter of 12 cm and a thickness of 1.1mm was molded at a mold temperature of 125° C., a resin melt temperatureof 360° C. and a mold clamping force of 35 tons.

[0101] On the other hand, as the resin stamper 140, using an olefinresin (ZEONEX-E28R fabricated by Zeon Corporation), the resin stamper140 having an outer diameter of 13 cm and a thickness of 1.0 mm wasmolded at a mold temperature of 125° C., a resin melt temperature of370° C. and a mold clamping force of 35 tons. An olefin resin was usedas a material of the resin stamper 140 in view of ensuring of the easeof removal after transcription of the second information recording layer114, that is, in view of the characteristic that adhesion between thefirst light transmission layer 110 and the resin stamper 140 is smallerthan that between the substrate 104 and the first light transmissionlayer 110, and the characteristic that a light transmittance at theabove-described wavelength from 300 nm to 400 nm is 50% or more.

[0102] The outer peripheral shape of the substrate 104 is such that athin-walled portion (inclination) starts from the position 1.0 mm awayfrom the outer peripheral edge and a thickness of the thin-walledportion (the inclined plane 135) at the outer edge is 1.0 mm. The outerperipheral shape of the resin stamper 140 is such that a thickenedportion starts at the position 1.0 mm away from the outer peripheraledge of the corresponding optical disc substrate and a thickness of theconvex portion 140A at the outer edge is 1.1 mm.

[0103] When the above-described optical disc 102 was fabricated by usingthese substrate 104 and resin stamper 140, the raise 160 in the outerperipheral portion did not project beyond the surface of the secondlight transmission layer 116, corresponding to the information recordingarea S2, resulting in a concave state of 20 μm.

[0104] In this embodiment, since the first and second light transmissionlayers 110 and 116, through which the laser beam pass, are formed tohave thicknesses of about 20 μm and 90 μm, respectively, coma aberrationcan be kept small even if the numerical aperture NA is increased. Forimprovement in recording density for each layer by increasing the NA ofthe laser beam, larger capacity can be realized as compared with adouble-layered type in which the laser beam is irradiated on the side ofthe molded substrate 30 (the type in FIG. 13). Moreover, as comparedwith a conventional similar single-layered type (the type in FIG. 14),the capacity can be increased for the increase in the number of layers.

[0105] Moreover, even in the case where the second light transmissionlayer 116 is formed by spin coating, the raise 160 on the surface of thecoating can be prevented from being noticeable as a raise on the surfaceof the disc, owing to the bending shape of the outer peripheral portion.Therefore, means for smoothing the upper surface of the second lighttransmission layer 116 is not particularly needed. Therefore, largercapacity and lower cost can be simultaneously achieved in a simplemanner.

[0106] In the above-described embodiment, the thicknesses of the firstand second light transmission layers 110 and 116 are about 20 μm and 90μm, respectively. However, the thicknesses of the first and second lighttransmission layers according to the present invention are not limitedthereto. In view of the reliability of fabrication of optical discs,however, the thicknesses of these light transmission layers are requiredto be several μm or more at the minimum. Moreover, in view of an opticalsystem with a higher NA, it is preferred to keep the thicknesses toabout 40 μm and 170 μm at the maximum, respectively. More preferably,the thicknesses are 30 μm and 130 μm or less, respectively.

[0107] Moreover, in the above-described embodiment, the outer peripheralportion of each layer is bent, so that even in the case where theuppermost second light transmission layer is formed by spin coating,attention has been paid not to bring about any particularinconveniences. However, in the case where the optical disc according tothe present invention is fabricated, the bending structure is notnecessarily essential.

[0108] As described above, conventionally, some methods or techniquesfor smoothing the upper surface of the resin coating by spin coatinghave already been proposed. For the formation of the second lighttransmission layer, the present invention does not forbid to use, forexample, any one of these propositions.

[0109] Moreover, although the outer diameter of the resin stamper 140 is13 cm in this example, a similar result can be obtained with the sameouter diameter as that of the substrate 104, that is, 12 cm.

EXAMPLE 2

[0110] In accordance with the experiment, which was carried out by theinventors for comparison in parallel with the experiment in theabove-described Example 1, the optical disc (102) was fabricated underthe same conditions as those of the above-described Example 1 exceptthat the combination was formed by the substrate having a plane outerperipheral portion (on which the curved pattern is not formed) (104) andthe resin stamper (140). As a result, the raise (160) at the outermostperiphery was in a convex state at 60 μm beyond the surface of thesecond light transmission layer 116 corresponding to the informationrecording area (S2). This state needs some sort of post-treatmentbecause there is a possibility that the optical disc (102) and thepickup of the optical system collide with each other in this state.

[0111] However, as shown in FIG. 8, for example, if a technique forforming the optical disc 202 itself to have slightly larger size R1 thanthat of a final product and thereafter cutting an outer peripheralportion AB to obtain predetermined size R2 is employed, thisinconvenience can be eliminated.

[0112] Moreover, the surface can alternatively be smoothed by placing aglass plate 221 having a smooth surface (see a phantom line in FIG. 8)as a dummy stamper on the upper surface of the second light transmissionlayer 216. Further alternatively, bonding using a resin sheet may beemployed.

[0113] The optical disc according to the present invention has anapparently advantageous high added value over any conventionaltechniques for increasing the capacity. Moreover, the first lighttransmission layer can be fabricated without any technical problems.Therefore, there is substantially little increase in cost. Thus, even ifthe same measure as that in the conventional case is taken only for thesecond light transmission layer, a burden on this in view of cost is“relatively” remarkably reduced.

[0114] Thus, for example, in the case where a so-called top-gradeversion optical disc, for which higher reliability should be ensured, isto be fabricated, another method may appropriately be used for atreatment of the second light transmission layer.

[0115] The structure of each layer in FIG. 8 is similar to that in theprevious embodiment except the presence/absence of the curved pattern.Therefore, the same or similar parts in the drawing are simply denotedby the reference numerals containing the same last double-digit numbersas those of the reference numerals in the previous embodiment, and theoverlapping description thereof is omitted.

[0116] Furthermore, in the above-described embodiment, the inclinedplane 135 is formed not only on the upper surface of the first lighttransmission layer 110 but also in the outer peripheral portion of theupper surface of the substrate 104. However, as shown in FIG. 9, if onlythe biased portion (inclined plane) 137 is formed in the outerperipheral portion of the upper surface of the first light transmissionlayer 110, the raise 160 can be prevented from being generated on theupper surface of the second light transmission layer 116 under someconditions. Therefore, if only the biased portion is formed at least inthe outer peripheral portion of the upper surface of the first lighttransmission layer 110, it is not necessarily required to form a biasedportion in the outer peripheral portion of the underlying substrate 104Ain some cases.

[0117] Moreover, the shape of the biased portion is not necessarilylimited to a simple inclined plane. In summary, in view of the object ofpreventing the raise in the outer peripheral portion of the spin coatedupper surface, it is apparent that a depth or a shape of the biasedportion can be determined, for example, based on the results of theexperiment which is conducted in order to smooth the spin coated surfaceas much as possible.

[0118] On the other hand, in the present invention, as means forsmoothing and shaping the upper surface of the resin coating layer whichis required to be formed thin, a further information recording layer isformed on the upper surface and a function of the stamper is utilized.Therefore, a multilayered recording medium having three or more layerscan be easily fabricated by successively performing this technique.

[0119] For example, in order to fabricating a triple-layered opticalrecording medium, as shown in FIG. 10, a substrate 304; a firstinformation recording layer 308 formed on the substrate 304; a firstlight transmission layer 310 formed of a resin to have a thickness of 40μm or less on the first information recording layer 308; a secondinformation recording layer 314 formed on the first light transmissionlayer 310; a second light transmission layer 316 formed of a resin tohave a thickness of 40 μm or less on the second information recordinglayer 314; a third information recording layer 322 formed on the secondlight transmission layer 316; and a third light transmission layer 324formed of a resin to have a thickness of 170 μm or less on the thirdinformation recording layer 322, are sequentially provided so that thelaser beam La is irradiated on the side of the third light transmissionlayer 324 to allow recording and/or reading of an information signalto/from the first, second and third information recording layers 308,314 and 322.

[0120] For transcription of each of the information recording layers,similar resin stampers 340 and 350 to the above-described stamper 140are respectively used. An inclined plane 339 is also formed on the uppersurface of the second light transmission layer 316. A raise 360 isabsorbed by the presence of the inclined plane 339.

[0121] Furthermore, in a more general expression, in order to fabricatean n-layered optical recording medium including three or more layers, asubstrate; a first information recording layer formed on the substrate;a first light transmission layer formed of a resin to have a thicknessof 40 μm or less on the first information recording layer; a secondinformation recording layer formed on the first light transmissionlayer; a second light transmission layer formed of a resin to have athickness of 40 μm or less on the second information recording layer; .. . ; an n-th information recording layer formed on an (n−1)-th lighttransmission layer; and an n-th light transmission layer formed of aresin to have a thickness of 170 μm or less on the n-th informationrecording layer, are sequentially provided, where n is an integer of 3or larger, so that the laser beam is irradiated on the side of the n-thlight transmission layer to allow recording and/or reading of aninformation signal to/from the first, second . . . and n-th informationrecording layers.

[0122] As in the case of two layers, even in the case of a multilayeredstructure, if a structure is such that an outer peripheral portion of anupper surface of the (n−1)-th light transmission layer is biased towardthe lower layer, a raise, which is generated at the formation of theuppermost n-th light transmission layer by spin coating, can beprevented from being noticeable. However, in the case where a thicknessof the (n−1)-th light transmission layer is too small (to absorb theraise), the outer peripheral portion is biased from the lower layer asin the above-described embodiment. If it is biased from the lower layer,a large bias (if required, a bias larger than the thickness of the(n−1)-th light transmission layer) can be formed on the upper surface ofthe (n−1)-th light transmission layer even if the (n−1)-th lighttransmission layer is thin. Accordingly, the raise generated at theformation of the n-the light transmission layer can be sufficientlyabsorbed.

[0123] Even in the case where the optical disc having three or morelayers on one side, an allowable value of a thickness of each lighttransmission layer for disc formation is 170 μm or less, preferably, 130μm or less for the n-th light transmission layer, 40 μm or less,preferably, 30 μm or less for the other light transmission layers.Therefore, there is no particular difference from the case of twolayers.

[0124] In the case of three or more layers, however, if each of thelight transmission layers is too thick, the position where the firstinformation recording layer is formed is far from the end face on thebeam irradiation side. Therefore, it is qualitatively preferred that athickness is smaller than that of a double-layered optical disc. It canbe considered that values smaller than the above-described values by 30to 70% are practically allowable values, respectively.

[0125] In the last place, the present invention can also be modified asshown in FIG. 11. Namely, two multilayered recording media (402A and402B) formed in the above-described manner are prepared. They are bondedwith each other so that respective substrates 404A and 404B face eachother inward. As a result, a double-faced multilayered optical disc 402,in which the laser beam La is irradiated on both faces of the disc 402so as to allow recording and/or reading of an information signal on/fromthe respective sides.

[0126] If the substrates 404A and 404B are formed slightly thin (about0.6 mm), a thickness of the optical disc 402 as a whole can be kept atthe same as that of the above-described single-faced optical disc.

[0127] In the double-faced multilayered optical disc 402, in contrast toa conventional double-faced type optical disc, the access is made toeach information recording layer from the side of the thin lighttransmission layer. Therefore, a recording density for each layer can bekept high at the level of the optical disc in FIG. 14. Moreover, themultilayer structure is achieved on one face. In addition, themultilayer structure is formed on both faces. Therefore, an optical discof the largest capacity class at the present can be constituted.

[0128] Since the structure of each layer is similar to that of theprevious embodiment, the same or similar parts in the drawing are simplydenoted by the reference numerals containing the same double-digitnumbers as those of the reference numerals in the previous embodimentand the overlapping description thereof is omitted.

INDUSTRIAL APPLICABILITY

[0129] According to the present invention, in order to form a lighttransmission layer thin to realize a much higher NA of a laser beamwithout any problems, smoothing of the upper surface of the lighttransmission layer, which is conventionally a bottleneck in fabrication,is realized by a reversal and progressive idea of forming a furtherinformation recording layer. Therefore, the excellent effects ofobtaining an optical disc, in which larger capacity of an opticalrecording medium is realized while keeping cost down, can be obtained.

1. An optical recording medium comprising: a substrate; a firstinformation recording layer formed on the substrate; a first lighttransmission layer formed of a resin on the first information recordinglayer, the first light transmission layer having a thickness of 40 μm orless; a second information recording layer formed on the first lighttransmission layer; and a second light transmission layer formed of aresin on the second information recording layer, the second lighttransmission layer having a thickness of 170 μm or less, wherein a laserbeam is irradiated on a side of the second light transmission layer soas to allow recording and/or reading of an information signal on/fromthe first and second information recording layers.
 2. The opticalrecording medium according to claim 1, wherein a portion of the uppersurface of the first light transmission layer, corresponding to an outerradial portion of the information recording area of the secondinformation recording layer, is biased toward the substrate from asurface corresponding to the information recording area.
 3. The opticalrecording medium according to claim 2, wherein the bias of the uppersurface of the first light transmission layer is inclined toward thesubstrate in such a manner that the thickness of the first lighttransmission layer is gradually reduced in an outer radial direction. 4.The optical recording medium according to claim 2 claim 2 of 3, whereina portion of the upper surface of the substrate, corresponding to anouter radial portion of the information recording area of the firstinformation recording layer, is biased toward inside of the substratefrom the surface corresponding to the information recording area.
 5. Theoptical recording medium according to claim 4, wherein the bias of theupper surface of the substrate is inclined toward the inside of thesubstrate in such a manner that a thickness of the substrate isgradually reduced in an outer radial direction.
 6. The optical recordingmedium according to claim 1, wherein each of the first and second lighttransmission layers is an UV curable resin.
 7. An optical recordingmedium sequentially comprising: a substrate; a first informationrecording layer formed on the substrate; a first light transmissionlayer formed of a resin on the first information recording layer, thefirst light transmission layer having a thickness of 40 μm or less; asecond information recording layer formed on the first lighttransmission layer; a second light transmission layer formed of a resinon the second information recording layer, the second light transmissionlayer having a thickness of 40 μm or less; . . . ; an n-th informationrecording layer formed on an (n−1)-th light transmission layer; and ann-th light transmission layer formed of a resin on the n-th informationrecording layer, the n-th light transmission layer having a thickness of170 μm or less, wherein a laser beam is irradiated on a side of the n-thlight transmission layer so as to allow recording and/or reading of aninformation signal on/from the first, second . . . and n-th informationrecording layers, and wherein n is an integer of 3 or larger.
 8. Theoptical recording medium according to claim 7, wherein at least aportion of the upper surface of the (n−1)-th light transmission layer,corresponding to an outer radial portion of the information recordingarea of the (n-i)-th information recording layer, is biased toward thesubstrate from a surface corresponding to the information recordingarea.
 9. An optical recording medium characterized in that two opticalrecording media according to claim 1 are prepared and bonded to eachother so that their substrates face inside, whereby the laser beam isirradiated on both sides of the disc to allow recording and/or readingof an information signal on/from the respective sides.